1. Field of the Invention
This invention relates to corona discharge treatment of plastic films or preformed hollow articles. More particularly, it relates to corona discharge treatment of selected surfaces of such films or hollow articles in order to prevent them from blocking, especially thermal blocking which results from exposure to a temperature close to the melting point of the plastic.
Blocking can be defined as the tendency of touching film surfaces to adhere to each other or to resist separation. Blocking is caused by either or both of the following: (a) extremely smooth film surfaces, allowing intimate contact and nearly complete exclusion of air, and (b) pressure- or temperature-induced fusion of the surfaces in contact, or both. Blocking develops under a variety of temperatures and pressures. Blocking is a function of time and it may arise from processing, use, or storage. For a method of measuring blocking, see ASTM method D-1893-67.
In practice, film layers are normally separated by a sliding and/or a lifting force. The lifting component is resisted by blocking forces while the sliding component is resisted by frictional forces. Blocking can be measured as the perpendicular component of the force required to peel two film layers apart per unit width of the film specimens. The blocking and frictional forces are theoretically at right angles to each other.
Blocking of film layers in a finished film article such as a bag manifests itself by poor bag opening. The film layers forming the bag may be blocked so severely as to render the bag virtually impossible to open by hand or by equivalent mechanical means and therefore useless in automatic, or rapid manual, filling operations. At times, blocking between two adjacent film layers is so severe that the films appear as one continuous piece. If film is supplied from a blocked pay-off roll, difficulties in film processing through machinery and fabrication may result. If the film layers in the roll were blocked severely, pay-off action could actually come to a halt. A blocked tubular film layflat is difficult to slit and rewind as single sheets, or if the layflat is converted directly into e.g., bags, the block is incorporated into the finished bags. Thus, blocking generally results in excessive waste of film, time, and low yield of finished goods. The above examples are meant to be only a partial listing of difficulties arising from blocking of plastic films, but they serve to illustrate the need for effective methods of blocking prevention.
In this application, the term "biomedical" is used to mean suitable and/or adapted for use in biological, medical and physical science. The terms "inert gas" or "noble gas" are used interchangeably to denote a gas selected from Group VIII of the Periodic Table of Elements.
The term "energize" is used to mean ionize and/or excite. It is used in reference to gas atoms.
The term "corona discharge in an inert atmosphere" or "corona-type discharge" shall be used to designate a process equivalent to that termed "corona discharge" in the art, the only difference being that the medium in wich the discharge of this invention is struck, consists essentially of a noble gas, not air.
The foregoing was a discussion of blocking in plastic films in general and it is not confined to blood bags or the biomedical field.